Structural Provinces of the Ross Ice Shelf, Antarctica

Home , Beardmore Glacier, King Glacier, Minna Bluff, Mulock Glacier, Nimrod Glacier, Ross Ice Shelf

Annals of Glaciology 58(75pt1) 2017 doi: 10.1017/aog.2017.24 88 © The Author(s) 2017. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons. org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited. Structural provinces of the Ross Ice Shelf, Antarctica

Christine M. LEDOUX,1 Christina L. HULBE,2 Martin P. FORBES,2 Ted A. SCAMBOS,3 Karen ALLEY3 1Department of Geology, Portland State University, Portland, OR 97215, USA 2School of Surveying, University of Otago, Dunedin, New Zealand. E-mail: [email protected] 3National Snow and Ice Data Center, University of Colorado Boulder, Boulder, CO, USA

ABSTRACT. The surface of the Ross Ice Shelf (RIS) is textured by flow stripes, crevasses and other features related to ice flow and deformation. Here, moderate resolution optical satellite images are used to map and classify regions of the RIS characterized by different surface textures. Because the textures arise from ice deformation, the map is used to identify structural provinces with common deformation history. We classify four province types: regions associated with large outlet glaciers, shear zones, extension downstream of obstacles and suture zones between provinces with different upstream sources. Adjacent provinces with contrasting histories are in some locations deforming at different rates, suggest- ing that our province map is also an ice fabric map. Structural provinces have more complicated shapes in the part of the ice shelf fed by West Antarctic ice streams than in the part fed by outlet glaciers from the Transantarctic Mountains. The map may be used to infer past variations in stress conditions and flow events that cannot be inferred from flow traces alone. Keywords: crevasses, ice shelves, structural glaciology

INTRODUCTION both the ambient stress field and near-field modifications to Linear features observed at the surface of a flowing ice mass that field arising from the fractures themselves. Material prop- represent an integrated history along the flow trajectory. In an erties of the ice may modify its fracture toughness and thus ice shelf, that history includes variation in flow of tributary either promote or suppress propagation (Khazendar and glaciers, conditions at the coastline where the ice went others, 2007; Glasser and others, 2009; Hulbe and others, afloat, and processes within the floating ice (see Fahnestock 2010; Borstad and others, 2012). Most crevasses visible at and others, 2000; Glasser and others, 2009, 2015;Elyand the surface remain relatively short over most of the advective Clark, 2016). Linear features include both along-flow-oriented pathway, and only become long (and thus the failure planes flow stripes and crevasses whose presence and geometry are for large icebergs) in the near-front environment, an effect determined by principal stresses in the ice (e.g. McGrath and that can be related to both material properties and the inter- others, 2012; Colgan and others, 2016). Flow stripes at the action among nearby fractures (cf. Lazzara and others, 1999; surface can be traced back upstream to source regions and, Hulbe and others, 2010; Brunt and others, 2011). together with textures due to crevasses, can be used to identify Our primary interest is in the horizontal expression of frac- regions with common history and to infer deformation history ture propagation as a mixed-mode opening (mode I) and of the ice within each region. sliding (mode II) phenomenon and governed by conditions Surface features record ice shelf history in different ways. within the ice shelf. Once initiated, crevasses are advected Once formed, flow stripes and crevasses are transported in the flow field and may change orientation or shape toward the calving front. Because the flow field is neither spa- depending on the stress field encountered along the way. tially nor temporally uniform, the features experience spa- As passive features, crevasses may be rotated, elongated, tially and temporally varying stress and strain conditions. closed or otherwise modified by the viscous flow of the ice. This gives rise to time-varying deformation that may be inter- They may also be buried by new snow. Where crevasses preted in terms of past variability. For example, streaklines continue to propagate or are reactivated during advection, are surface undulations associated with flow variations in their shapes record stresses experienced along the path and grounded source ice (Glasser and others, 2015). Once perhaps also changes in the ice shelf (MacAyeal and afloat, these flow features become passive tracers and their Lange, 1988; Hulbe and Fahnestock, 2007). deformation relative to the modern flow field has in the The analysis presented here begins with a feature map of case of the Ross Ice Shelf (RIS), been used to infer past flow the RIS created using moderate resolution images of the ice variability (Jezek, 1984; Casassa and others, 1991; shelf surface. Crevasse geometries are classified in a scheme Fahnestock and others, 2000; Hulbe and Fahnestock, 2007). that emphasizes what they reveal about principal stress In the present contribution, we consider flow stripes and orientations. Next, structural provinces (Jackson and Kamb, crevasse patterns together, in order to consider deformation 1997; Herzfeld and others, 2013) within the ice shelf are clas- in the ice shelf more completely. Glacier ice undergoes sified according to pervasive crevasse patterns within regions. brittle fracture at sufficiently large stresses (cf. Rist and Comparison with remotely sensed, present-day strain rates, others, 1999). Fractures nucleate in the subsurface (Nath principal stresses, and ice thickness allow us to discuss ways and Vaughan, 2003) and when stress intensity at a crack in which structural provinces may be used to learn about tip is large enough, the crack will propagate in response to both past and present conditions on the ice shelf.

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DATA rate results, we prefer the Landsat-8-derived field for its sharper and more easily managed artifacts. Uncertainty in − Image mosaics the Landsat-8-derived velocity field is 5 m a 1 and the propa- Clear feature geometries that are straightforward to digitize gated uncertainty on the effective strain rate, using gradients − are required for mapping crevasse patterns and regional computed on a 750 m regular grid, is 0.008 a 1. This is surface textures. We are interested in groups of features similar to errors reported for MEaSUREs. The estimated with similar orientations, which produce textures at the ice error does not capture noise in the effective strain rate field. _ surface that persist over tens of km rather than finer, sub- The 1 − σ standard deviation in the ee field downstream of km scale, details. While automated approaches exist (Ely the large outlet glaciers discussed in a later section is − and Clark, 2016), we hand-digitize features with the inten- 0.016 a 1. σ0 ¼ σ ð = Þσ δ tion of developing familiarity with the textures and the mech- Deviatoric stresses are computed as ij ij 1 3 ij ij δ δ σ anical settings in which they are produced. These objectives in which ij is the Kronecker . The stress tensor ij is esti- e make moderate resolution optical images suitable for our mated using the finite strain rates ij and the inverse form of work. the generalized flow law for ice Two image mosaics are the primary data sources for our map: the first epoch of the MODIS (Moderate Resolution σ ¼ e_ð1=nÞ1e_ ð Þ ij B e ij 2 Imaging Spectroradiometer) Mosaic of Antarctica (MOA; Haran and others, 2005) and the Landsat Image Mosaic of in which the exponent n is equal to 3. We use a value of 760 − Antarctica (LIMA; Bindschadler and others, 2008). The first kPa a 1/3 for the temperature-dependent rate factor B, epoch of the MOA is a composite of 260 individual equivalent to a depth average temperature of 247 K (Van MODIS images acquired between 20 November 2003 and der Veen, 1999, p. 16). Principal stress directions and magni- 29 February 2004 with an effective resolution of 125 m. tudes are computed as the eigenvectors and eigenvalues of The LIMA, which has a resolution of 30 m and covers the deviatoric stress tensor. The uniform rate factor is very ° ice shelf region north of about 82 S, is a composite of simple, but our interest is in the eigenvector directions nearly 1100 images acquired by the Landsat ETM+ rather than the values. (Enhanced Thematic Mapper Plus) sensor between 1999 and 2003. Because the MOA composite amplifies shadows, low-relief features like along-flow lineations, Ice surface elevation sagging snow bridges, and regional textures arising from We use the Digital Elevation Model-derived from measure- snow-covered features are readily apparent in the mosaic. ments made by the Geoscience Laser Altimeter (GLAS) We thus use the MOA to make an initial mapping of the (DiMarzio, 2007) to supplement the discussion of ice shelf whole ice shelf surface and turn to the higher resolution provinces. In general, thicker ice floats higher in the ocean LIMA and some individual Landsat 8 scenes, obtained via surface but we do not convert the surface elevation into the USGS EarthExplorer (earthexplorer.usgs.gov), to resolve thickness because we lack knowledge of spatial variation ambiguities regarding boundaries of some textured regions. in snow density and because this is not necessary for the intended use of the data.

Velocity, strain rates and stresses METHODS Glaciological strain rates and stresses, computed using remotely sensed surface velocity, are used to support inter- A wealth of surface features are visible in the MOA and LIMA pretation of the feature maps. We compared two velocity mosaics, including streaklines, crevasses and through-cutting products, the InSAR-derived MEaSUREs data set (Rignot rifts. We began by digitizing prominent streaklines and cre- and others, 2011) and a Landsat-8-derived velocity com- vasses as individual linear traces observed in the MOA puted by correlating grey scale patterns in small sub-scenes (Fig. 1). Crevasses appear in the images either as sharp, between sequential images collected between 1 October sunlit/shadowed faces or as shadows cast within sagging 2014 and 31 March 2015 (Fahnestock and others, 2016). snow bridges (Merry and Whillans, 1993). This means that The former covers the entire RIS, while the latter has a south- our approach is blind to features that have yet to open ern limit of 82.65° S. across the fracture plane. Strain rates are approximated as horizontal gradients of The crevasse map is not exhaustive. We digitized major the two dimensional velocity field. We compute effective features and groups of features that facilitate our ambition _ strain rates ee as the second invariant of the strain rate tensor to create a structure map of the ice shelf. The first draft of the map was checked by comparison with the LIMA and indi- 1 vidual Landsat 8 scenes. Crevasses with similar geometries e_ 2 ¼ e_ e_ ð1Þ e 2 ij ij were then grouped by type, and a relative sense of time was incorporated into the scheme by distinguishing in which the subscripts i,j represent three orthogonal direc- between initial crevasse geometries that represent the early tions. Vertical shearing (the vertical gradient of horizontal part of a fracture’s history and thus are simple to interpret velocity components) is negligible in the floating ice shelf and advected crevasse geometries that arise from additional and the vertical normal strain rate is calculated from the hori- propagation along the advective path. _ _ _ zontal divergence ðezz ¼exx eyyÞ. Artifacts and errors in both data sets are amplified by the gradient calculation. These appear as linear features asso- Initial geometries ciated with image edges, circuitous shapes associated with 1. Transverse geometries form where grounded ice goes clouds and as noise. Comparing the two effective strain afloat and longitudinal stress gradients are large. The

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Fig. 1. The Ross Ice Shelf as it appears in the 2005 MODIS Mosaic of Antarctica (MOA). Areas described in more detail are (a) Crary Ice Rise, (b) coast of the Transantarctic Mountains between Beardmore and Nimrod Glaciers, (c) Minna Bluff.

initial geometry may be preserved during advection in a 3. Obstacle-Shear geometries form where floating ice moves relatively uniform velocity field or rotated and elongated past features such as ice rises, coastal headlands and by gradients in the velocity field, even in the absence of outlet corners of fjords. Stresses may be very large in additional propagation. these situations, resulting in vertical propagation 2. Margin-Shear geometries form in grounded ice due to through the full thickness of the ice and opening to form simple shear in a relatively narrow zone near a sidewall large rifts. The surface expression of these crevasses and (Fig. 2). Details in the shapes of margin-shear crevasses rifts persist for long distances downstream (100s of kms), differ between glaciers and ice streams (see Merry and and the geometries often deform, rotating and elongating Whillans, 1993; Colgan and others, 2016), but the interest as they advect toward the shelf front. here is the broad scale surface texture rather than details. The overall orientation is upstream pointing, at an angle to the margin. Propagation during advection Crevasses may maintain their initial geometry as they advect, either continuing to propagate or becoming passive features. They may also experience new episodes of propagation where the principal stress directions change, either due to advection or to near-field effects that arise as crevasse length grows. In such situations, the stress intensity is large enough to promote propagation but the initial geometry is poorly aligned with the local principal stress directions, so a new geometry emerges. 1. Shear to Transverse geometries develop when crevasses associated with shear past a lateral margin advect into a stress field favoring propagation transverse to flow (LeDoux, 2007; Hulbe and others, 2010). Long transverse rifts in the RIS originate as either margin-shear or obstacle- shear crevasses that subsequently propagate transverse to the ice flow direction in response to compression in the Fig. 2. Crevasses propagate in the most compressive principal stress direction. Shear stresses along the margins of glaciers and coastlines across-flow direction. produce left and right margin-shear zones in which the sense of 2. Secondary crevasses may develop at both high and low shear is as would be the case in other geologic settings. The angles to the planes of larger crevasses in regions charac- resulting crevasses are upstream pointing, in the direction of terized by large shear strain rates. The crevasses form in maximum compression. response to relatively large shear stresses around the

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ends of the primary crevasses and are not cross-cutting. 4. Suture Zone provinces are characterized by surface tex- Both mode I and mode II fractures may form in situations tures that lack a clear preferred orientation. The simplest like this (Martel and Boger, 1998; Kim and Sanderson, sutures form downstream of obstacles like ice rises. 2006). More complicated, composite suture zones form where 3. Tip Interaction generates distinctive geometries indicative several small outlet glaciers merge and turn together of near-field modification of the stress field (Cotterell and into the ice shelf. Rice, 1980; Pollard and Aydin, 1988). Crevasse tips may propagate in either a converging or diverging sense depending on the magnitude and orientation of the prin- PROVINCE MAP OF THE RIS cipal stresses and on the locations and geometries of other Our province map builds upon past analyses of the RIS nearby fractures. (Fahnestock and others, 2000; Hulbe and Fahnestock, 4. Widening geometries arise when stress conditions favor- 2007) by synthesizing longitudinal flow features together able to propagation exist but growth in the propagation with regional crevasse patterns and the structural provinces direction is inhibited. This may occur at structural implied by them. For ease of examination, we present the boundaries where ice properties affect the fracture map in two layers (Figs 3 and 4). toughness of the ice (Hulbe and others, 2010). In such As defined here, the structural provinces are flowbands, or circumstances, crevasses widen across the fracture segments of flowbands, with a distinct surface texture from plane. which the deformation history of the underlying ice can be inferred. Interpretation of surface texture is more straightforward in some regions than in others. Some textures and asso- Provinces ciated provinces persist over long distances, while elsewhere, initial textures are quickly lost and in some Structural provinces are adjacent regions with significantly cases replaced by a new texture. The boundaries on the different geologic structures (Jackson, 1997, p. 631). map are approximate and we have chosen to make a classi- Following this, we define ice shelf structural provinces to fication only where the evidence is clear. be regions characterized by distinctive surface texture indi- Many Transantarctic Mountains (TAM) outlet glaciers cative of a common deformation history. That history, in experience an abrupt change in flow direction as they turn, implies that ice material properties may vary across enter the ice shelf. As a result, crevasse patterns due to province boundaries. Because flow bands emerging from shear around outlet corners can be short lived, as ice glaciers and ice streams are subject to new stress fields as turning past a corner enters into a region with different prin- they advect through the shelf, structural provinces should cipal stresses. Where many small outlet glaciers merge, we not necessarily replicate flow band boundaries. Here, ice classify one broad suture zone characterized by a mottled shelf structural provinces are identified as regions with surface and no clearly preferred orientation of surface characteristic crevasse patterns and surface textures. features. Surface textures are most clearly expressed in the MOA Structural provinces are more readily identified in the because image stacking amplifies shadows cast by subtle western part of the RIS than in the east. This is due to the rela- features. Province boundaries were checked by compari- tively more complex grounding zone in the east and the long son with the LIMA and Landsat 8. Examples are presented history of ice stream discharge variability and ice rise forma- in the next section. tion in that region (see Hulbe and Fahnestock, 2007). We do 1. Transverse Crevasse provinces are characterized by linear not use streaklines to connect similar provinces along the features oriented approximately across flow. They contain flow direction because gaps may (or may not) represent ice that experienced significant longitudinal stretching past changes in ice shelf flow or spatial variation in the (transverse compression) and vertical shortening down- ambient stress field. stream of a grounding line. This type of texture is most clearly identified and persistent in the eastern region of the RIS, where the shelf is fed by ice streams from the Crary Ice Rise West Antarctic Ice Sheet. The region around Crary Ice Rise (CIR) is complicated by its 2. High Discharge provinces are characterized by rough geometry and by a history of ice flow variability (Fig. 5). surface texture downstream of large outlet glaciers. This Upstream-pointing crevasses due to shear form narrow roughness is due to cross-cutting crevasses and across- bands along its shorelines. Other, more prominent, textures flow undulations developed when the ice was grounded. characterize the surrounding area. Outboard of the ice Longitudinal flow stripes developed in the grounded ice rise’s northern shore, a broad region characterized by trans- may splay out or be rotated as the glacier flows into the verse crevasses is associated with discharge across the ice shelf. grounding line of the Whillans Ice Stream (WIS) ice plain. 3. Left and Right Margin-Shear provinces are characterized Between these we identify a province containing ice that by well-organized, initially upstream-pointing surface was once in an ice stream shear zone. Downstream, older features that persist for long distances downstream. crevasses appear to be reactivated with complicated propa- Margin-shear provinces arising from outlet glaciers may gation directions that imply near-field modification of the retain their original character, be rotated or overprinted stress field due to interactions among adjacent fractures. as a glacier discharges into the ice shelf. New shear Textures are less clear to the south of the ice rise, until the zone provinces may develop where floating ice flows ice shears by its downstream end, generating a distinctive around a headland or parallel to a coastline. We use the set of what we have called an obstacle-shear initial geometry. terms left and right to indicate the sense of shear, in the These crevasses persist for a long distance downstream, horizontal plane, where the texture originated. carried along by the ice flow.

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Fig. 3. Streaklines and crevasses digitized from the MOA, grouped and color coded by geometry.

Fig. 4. Structural provinces identified according to surface texture in various flow bands within the RIS.

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Fig. 5. The area around Crary Ice Rise, at the dowstream end of the Whillans Ice Stream ice plain. (a) Features as observed in the MOA. (b) Digitized streaklines and crevasses, with interpreted structural provinces.

Large open rifts form near the downstream end of CIR and Minna Bluff a narrow suture zone forms between these. Crevasses and An extensive set of crevasses and open rifts are produced rifts along the flanks of the suture change shape as they where ice outlet glaciers flowing into the RIS turns north advect toward the shelf front and in some locations new, and shears past Minna Bluff (Fig. 6). Crevasses originating smaller fractures are observed between the older, larger fea- at the upstream end of the bluff cut across snow-covered cre- tures. These crevasses are neither uniformly distributed nor vasses that originated far upstream. The new crevasses propa- uniformly distorted and this is consistent with past disruption gate eastward until they arrest near the boundary of a of discharge from WIS (Hulbe and Fahnestock, 2007). province containing merged suture zones from the Skelton,

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Fig. 6. The eastern side of Minna Bluff and adjacent RIS. (a) Digitized streaklines and crevasses, with interpreted structural provinces. (b) Detail of a Landsat 8 panchromatic band image, path 224 row 128, acquired 11/15/2016, courtesy of the US Geological Survey with Landsat-8-derived surface speed. The white star marks the approximate location of the British National Antarctic Expedition’s ‘depot A’. (c) Principal stress directions and approximate magnitudes from surface velocity plotted over the Landsat 8 image. Red indicates the compressive direction and black indicates extension.

Evtimov and Mulock Glaciers. As the initial crevasses round TAM coastline between beardmore and Nimrod the corner and move past the bluff, a second episode of Glaciers propagation develops. Secondary fractures with the appear- Both small and large outlet glaciers discharge ice with clearly ance of exceptionally large horsetail fractures and wing identifiable initial crevasse patterns into the RIS along the cracks also form in this zone. The primary and secondary cre- front of the TAM. The trunks and margins of larger glaciers vasses are all well aligned with principal stresses derived tend to persist as individual provinces while ice from from the observed velocity. The crevasses eventually rotate smaller glaciers is more likely to experience texture over- into downstream-pointing orientations and become snow printing or to merge into a suture zones as it joins the ice covered along the boundary between the McMurdo and shelf. The coastline between Beardmore and Nimrod Ross Ice Shelves (Whillans and Merry, 2001). Glaciers provides examples of both situations and demon- Active segments of the Minna Bluff crevasses align well with principal stress directions computed from the Landsat- strates several fates for glacier shear zones once they enter 8-derived velocities. Initial crevasse geometries at the the shelf (Fig. 7). upstream end of the region are aligned with the most com- Ice discharging from Beardmore Glacier turns sharply pressive principal stress direction there. As the crevasses north around Mount Hope as it enters the RIS. Ice from the advect downstream, their orientations change such that the glacier trunk is patterned by upstream-oriented linear fea- tips are poorly aligned with the stress field and secondary cre- tures that appear to be the continuation of heavily crevassed vasses begin to propagate in the direction of compression. flow stripes on the glacier surface. Ice from the left margin of Farther downstream, snow-covered crevasses are rotated to the glacier advects downstream into a shear zone province, downstream-pointing directions and experience compres- while on the right margin, an unknown basal obstruction dis- sion orthogonal to their trace. The first direct measurement rupts the flow and produces a band of transverse crevasses in of ice shelf motion was made in this area, when members its wake. The headland of Lands End Nunataks produces a of Shackletons’ Nimrod Expedition remeasured the position similar band of transverse crevasses. In both cases, the of ‘depot A’,leftbythe1901–1904 British National Antarctic surface features rotate as they are advected downstream. – Expedition and found it to have moved about 500 yards a 1 Ice between the transverse crevasse provinces retains the – (457 m a 1) over the 6.5-year interval (Shackleton, 1909; surface texture indicative of margin-shear, but the features Debenham, 1948). The measurement was made by triangula- are rotated to a downstream-pointing direction. tion using the depot’s original alignment with two peaks on the The trunk of Lennox-King Glacier, northwest of – bluff. A remarkably similar value (425 m a 1) is obtained from Beardmore Glacier, is also characterized by a high discharge Landsat 8 image correlation. texture. In this case, the province associated with right lateral

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Fig. 7. Coastline of the TAM between Beardmore and Nimrod Glaciers. (a) Features as observed in the MOA. (b) Digitized streaklines and crevasses, with interpreted structural provinces.

margin-shear persists downstream, while the left lateral and thickness. This in turn may result in spatial variation far margin appears to merge into a suture zone with no domin- from the ice shelf margins (Borstad and others, 2012; ant orientation. Long shear-to-transverse crevasses form Hudleston, 2015). Ice thickness, and thus surface elevation, where the ice turns sharply left around Cape Maude and should be larger in bands emerging from glacier outlets Driscoll Point. The crevasses emerge from the left lateral than in the intervening suture zones. Strain rates should margin and arrest where their tips reach ice from the right vary relatively smoothly across the ice shelf, except where lateral margin. Another set of transverse crevasses propagate across-flow variations in ice properties either promote or sup- and arrest in a similar way in ice from Robb Glacier, just press deformation. We use effective strain rate and surface southeast of Nimrod Glacier. elevation to consider these possibilities in an exploratory way, focusing on the western part of the RIS where flow history is relatively simple (Fig. 8). Provinces and deformation in the ice shelf The outlets of large TAM glaciers, such as Byrd and Because they represent different upstream deformation his- Nimrod Glaciers, are easily identified in the effective strain tories, provinces imply spatial variation in ice properties, rate and surface elevation maps. The lateral margins of such as crystal preferred orientation (CPO), temperature grounded glaciers are characterized by relatively large

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Fig. 8. The western side of the RIS. (a) Color map of effective strain rate plotted over the MOA, using a log scale. (b) Surface elevation plotted over the MOA. MG, Mulock Glacier; SG, Skelton Glacier; MB, Minna Bluff; RI, Ross Island.

effective strain rates, but once the ice is afloat, the boundary Where the deformation rate is large, falls and then rises again that generated large shear stresses has been left behind. Fast in a particular zone, as downstream of Skelton, Mulock and deformation persists in some margin-shear provinces, for Byrd Glaciers, we infer that ice in the zone has a CPO that is example, at Byrd Glacier, while it dies away in others, for alternately well and poorly aligned for deformation in the example, at Nimrod Glacier. The relatively thin (low floating) ambient stress field. Ice in suture zone provinces is a compos- suture provinces are in some cases also characterized by ite of relatively thin, and fractured ice. Because the ice is relatively large effective strain rates. thinner (floats lower), basal meltwater rising from adjacent, Byrd Glacier appears to dominate the region of the ice deeper keeled high discharge provinces, may refreeze in shelf into which it discharges. Flow traces in the floating these zones. Where rifts have propagated through the full ice remain well aligned with the grounded glacier’s orienta- thickness of the ice, sea ice and marine ice may fill the tion more than 100 km downstream and the effective strain open area. Thus, these provinces may be deforming rela- rate remains large along the margins of the high discharge tively rapidly because the ice is thinner, warmer or does province for all of that distance. Where the province begins not have a CPO that makes it relatively stiff in any location to turn north, the fast deformation along its margins declines or orientation to the principal stress field. abruptly. It appears that ice in strongly sheared bands at the margins of the glacier continue to deform rapidly even without supporting sidewalls as long as they maintain the DISCUSSION same alignment relative to the principal stress directions. In his 1940 review of the global state of knowledge of Additional zones of relatively large deformation rate extend Antarctic glacier ice, Laurence M. Gould wrote ‘[p]robably both left and right of the Byrd Glaicer high discharge prov- no feature of Antarctic glaciology or geology has provoked ince. The zones have a feathery appearance, with filaments as much interest among explorers and has brought forth of slightly larger and slightly smaller deformation rate. The such great variety of opinions as to their origin as the shelf effective strain rate grows again near the front of the ice ice masses’ (Gould, 1940). Ice shelves continue to hold our shelf, in a zone comprised of the merged left and right attention, due in part to their role in mediating the progress margins of the Byrd and Mulock Glaciers. of ice sheet response to climate change (Mercer, 1978). Ice downstream from Mulock and Skelton Glaciers is The origin and composition of the RIS became a subject of characterized by patterns similar to that observed down- considerable debate from the moment sledging parties stream of Byrd Glacier. Margin-shear and suture zone ice made their first journeys across its surface and here too, gla- deforms more rapidly than ice from the glacier trunks. ciological interest persists. Edgeworth David and Raymond Initially large effective strain rates decline as the provinces Priestly, geologists on Shackleton’s 1907–1909 British turn northward into the shelf and then grow again farther Antarctic Expedition, proposed that the RIS was a composite downstream. Farther south along the TAM coast, ice dischar- containing ‘ribs’ of relatively stiff ice emanating from TAM ging from Beardmore and Lennox-King Glaicers turns into glaciers, draped by a surface accumulation of densifying the shelf over shorter distances and the traces of fast-deforming snow. The view was later discounted, in favor of local margins fade more quickly in the other examples, while ice snow accumulation (Gould, 1940). Modern satellite remote in the adjacent suture zone provinces continue to deform sensing of the ice shelf indicates that David and Priestly relatively rapidly. were more correct than later writers realized. All together, we conclude that these relationships are evi- The RIS is a composite of merged flowbands containing dence that material properties associated with structural pro- ice with a variety of deformation histories. Some bands are vinces affect ice deformation over large distances along flow. characterized by distinctive surface textures and where the

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interpretation is clear, we have mapped ice-shelf structural Spatially variable deformation history implies spatially provinces. Province configurations are relatively straightfor- variable ice properties (Hudleston, 2015). Ice in a province ward in the western part of the ice shelf, where flow traces characterized by sustained deformation of a particular type conform relatively well to the present day flow field (Hulbe will develop a CPO compatible with that history. Ice in trans- and Fahnestock, 2004), indicating that either TAM glaciers verse-crevasse and high-discharge provinces experienced experience relatively small amplitude flow variation over along-flow stretching and vertical shortening, while ice in time or that past variation is not well preserved (Hulbe and margin-shear provinces experienced simple shear. When Fahnestock, 2007). It may be possible to tease information ice with a CPO enters the ice shelf, that fabric is retained about past TAM glacier discharge variations, such as those until new conditions cause it to change, and this may make suggested by Jones and others (2016), out of finer resolution the ice either soft or stiff relative to stresses it experiences analysis of along-flow change in structural province charac- as it advects toward the calving front. We see evidence of teristics. In the east, even our conservative mapping indicates this throughout the western part of the RIS. a more complex past behavior than is currently known, such as variations in basal traction and grounding on ice plains (Fried and others, 2014; Winberry and others, 2014). ACKNOWLEDGEMENTS The structural map can support a variety of additional LeDoux was supported by NASA Cryosphere award number investigations. Structural provinces imply spatial variation NNX10AH81G. Hulbe was supported by New Zealand in ice mechanical properties and we have found some evi- Antarctic Research Institute award NZARI RFP 2014-2 dence of this. A more thorough investigation would require ‘Vulnerability of the Ross Ice Shelf in a Warming World’. velocity data sets with smaller errors and less noise. The velocity mapping was supported by NASA Cryosphere Alternatively, these properties could be sensed remotely, award number NNX16AJ88G. The digitized features and using seismological approaches (Maurel and others, 2015). province map will be archived as GIS shape files at the US Downstream changes in structural province type and over- National Snow and Ice Data Center and are also available printing of crevasse patterns may be attributed to temporal by request from Hulbe and Forbes. We thank two reviewers changes in stress regime along a flow band. 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